The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Magnetic disks are one example of a storage medium that is used in computer systems to store data. In general, magnetic disks are part of disk drives (i.e., hard disk drive (HDD)), which are often used as primary storage devices in computer systems. Typically, a disk drive includes one or more magnetic disks mounted on a spindle for rotation at high speeds by a servo motor. Additionally, a set of read-write heads are provided proximate to the magnetic disks to transfer data between the magnetic disks and the computer system. In general, the read-write heads are controllably positioned in relation to the magnetic disks in order to read and/or write data at different tracks of a magnetic disk. The tracks are, for example, concentric rings within the magnetic disk where data is stored.
Furthermore, each of the magnetic disks includes a single layer of information that includes the concentric rings. In general, the single layer comprises both stored data and position information that are interspersed with each other. The position information (e.g., position error signal (PES)) is information within the single layer that is used to position a read-write head over a desired track. The position information is provided in wedges (also referred to as Servo wedges) within the single layer that interrupt the tracks within which the data is stored. That is, the position information consumes space on the magnetic disks and is provided in a wedge shape that spans the magnetic disk. In general, each magnetic disk has a plurality of wedges spanning across and interrupting the tracks so that neither data nor the position information is continuously stored around the entire circumference of a track.
However, because the position information is not continuous throughout the single layer of the magnetic disk and occurs only at the wedges which are interspersed among the tracks, a location of a read-write head cannot be exactly known when the read-write head is not over a wedge. That is, the position of the read-write head can only be precisely known when the read-write head passes over a wedge that includes the position information. When the read-write head is over areas between wedges, the system cannot obtain position information for the head. Therefore, the head is subject to positioning error that results from external disturbances and random noise. The disturbances and random noise cause the head to move from a desired position. Thus, when the head moves into the next Servo wedge, the positioning error will be detected and corrected. Accordingly, spacing between tracks must account for inaccuracies because of the intermittent position information. Thus, not only do the wedges consume storage space within the single layer of the magnetic disk, but the wedges affect data density since inter-track spacing is generally increased to tolerate these errors.